La escuela primaria y la relación con los padres de familia

The identification of variables that are associated with maternal employment and that contribute to children’s overweight and obesity risk is critical in selecting potential

confounders that may bias the relationship between a mother’s work and her child’s weight status. Many of the reviewed articles include confounders after testing a large number of potential variables for significant univariate associations with childhood overweight and obesity,21,123 or have checked for confounding using the collapsibility criteria for

confounding in the absence of a priori reasoning.101 Several studies justify controlling for certain variables because previous articles in the field have done so,19,55 and acknowledge

that models may reflect over-adjustment due to the possibility that some controlled variables are in fact mediators.19 _{In order to avoid introducing bias into the present study, possible} confounding variables that are associated with both maternal employment and children’s weight status are reviewed below.

2.5.1 Maternal Age at Birth

A mother’s age at the birth of her child may influence her decision to participate in the workforce. Mothers over the age of 30 years are less likely to have returned to work at 2 months following their child’s birth compared to younger mothers.155 Compared to 25 to 29 year old mothers, Han et al.155 found that women under the age of 24 were significantly more likely to be working by 9 months following a child’s birth, and mothers 35 years and older were significantly less likely to be working.

Morrissey et al.54 found that maternal age at birth was associated with childhood obesity. Hawkins et al.21 have shown that maternal age at first live birth, rather than maternal age at the birth of the child under study, impacted overweight and obesity odds. Rooney et al.156 found that children’s obesity status varied by maternal age, but was not predicted by maternal age in regression models. Weng et al.157 _{did not find an association between maternal age at} birth and children’s odds of overweight and obesity.

2.5.2 Maternal Smoking During Pregnancy

Research has shown an association between smoking status and employment status.158–161 There is a higher rate of unemployment among smokers than non-smokers,161 and a greater proportion of both current and ever-smokers are unemployed compared to non-smokers.159 Additionally, a higher prevalence of smoking has been observed among long-term

unemployed individuals relative to all job-seekers.162 _{Unemployment has been shown to} significantly predict smoking status (adjusted OR=1.51, 95% CI=1.38-1.65).161

Studies have demonstrated that maternal smoking during pregnancy is independently associated with childhood overweight and obesity.101,103,163 Children whose mothers smoke during pregnancy are 1.43 and 2.06 times more likely to become overweight and obese, respectively.103 The exact mechanisms that link in-utero exposure to cigarette smoke and future weight are unknown. It has been suggested that maternal smoking may affect the

development of structures in the brain that are associated with reward processing, which may increase the preference for dietary fat intake in childhood and adolescence, ultimately leading to fat accumulation.164,165

2.5.3 Maternal Pre-pregnancy BMI

An association between weight status and employment has been demonstrated in the literature,166 with the relationship being especially strong for women rather than men.167,168 Despite more training and a greater number of job applications sent out, obese women may have worse employment outcomes than non-obese women.167 Time spent unemployed during working years is significantly associated with increased weight, and once unemployed, regaining employment is significantly less likely.169 Women who are obese are more likely to face employment discrimination compared to normal-weight women, resulting in difficulty getting hired or promoted.170 Compared to normal weight women, obese women earn lower annual salaries for the same position.170 Although the effect of employment on obesity has been less examined in the literature, there is some evidence that unemployment predicts weight gain.171

Compared to normal weight women, overweight women are at least 1.5 times more likely to have overweight children.21 Studies have demonstrated that maternal weight contributes to infant weight gain,172 and predicts preschooler and childhood overweight.91,173 Maternal pre- pregnancy obesity has been shown to increase the risk of pre-term birth,174 contributing to the delivery of low birth-weight babies who are more likely to become insulin resistant.175

Children who show catch-up growth, the early weight-gain observed among low-birth weight babies, are more likely to be fatter and have more central fat distribution compared to

children who do not exhibit post-natal catch-up growth.176

2.5.4 Size-for-gestational Age

There is evidence of an association between maternal employment and infant’s size for gestational age in the literature.177,178 Mothers who work irregular or shift-work schedules are at an increased risk of giving birth to a small-for-gestational age baby.177 Occupational conditions such as lifting loads179 and standing for extended periods180 also play a role in increasing the risk of babies being born small-for-gestational age. While associations

between occupational factors while working during pregnancy and birth outcomes have been demonstrated,181 _{some studies have shown separate associations for pre-term birth and low} birth weight but no significant associations for size-for-gestational age.181–183 _{Other indicators} of socioeconomic status such as educational attainment and income have been associated with size-for-gestational age outcomes.184

Studies have demonstrated that size-for-gestational age is predictive of children’s weight status. Small-for-gestational age (SGA) and appropriate-for-gestational age (AGA) infants who exhibit catch-up growth, as well as large-for-gestational age (LGA) infants without catch-down growth have higher BMIs as preschoolers.185 LGA infants without catch-up growth have been shown to have greater odds of childhood overweight and obesity.185 SGA infants have been shown to remain significantly shorter and lighter, while remaining taller and heavier was characteristic of LGA infants.186 Using anthropometric measures, LGA infants, but not SGA infants, continued to accumulate fat after 3 years of age.186

2.5.5 Marital Status

Employment has been shown to vary by mother’s marital status.187 In general, lone mothers are in greater need of income than women who are married and are supported by spousal earnings, requiring them to obtain employment in order to meet their financial needs. Results from Han et al.155 reveal little variation in the proportion of married, cohabiting, or single mothers who return to work in the early months following childbirth; however, by 9 months following a child’s birth, a slight gap forms, with single and cohabiting women more likely to be working than married women. Some studies have found the opposite to be true: a higher employment rate for mothers in two-parent families compared to lone-parent mothers.17 Recent data from Canada show that lone mothers with children under 3 years have a lower rate of employment (45.9%) than women with partners (66.5%), with the gap in the

employment rate diminishing with increasing child age.17 The large differences in rates of employment between lone and two-parent households with young children may be explained by the affordability of child care. In one study, the relationship between employment status and marital status varied by country.187 In the US, there were fewer differences in the

employment rate and employment status between lone mothers and married mothers, while in Germany there was a greater tendency of lone mothers to rely on full-time employment.187

The parental status of children is associated with children’s obesity status.188,189 Children of single-parent households are significantly more likely to be overweight or obese than children living with both parents.188–190

2.5.6 Maternal Education

Education and employment are closely associated.191 The positive association between education, occupation type and earnings has been demonstrated consistently in the literature.192 Individuals who hold university degrees have the highest employment rates compared to less educated individuals.191 Higher levels of educational attainment allow individuals to seek out better jobs that are associated with higher average earnings.193

Studies have shown that parental education is a strong predictor of childhood obesity.6,194 Maternal education, often used as an indicator for socioeconomic status195,196 has been shown to predict children’s overweight and obesity risk.197

2.5.7 Family Size (Number of Siblings in the Household)

Studies have demonstrated an association between maternal employment and family size.155,198 Results from Han et al.155 suggest that the number of children a woman has may influence her decision to participate in the workforce. Employment rates following women’s first and second births are notably higher than rates following third and subsequent births.155 The percentage of employed mothers at 9 months following birth is higher among mothers of first-born children compared to those with second-born and third-born children.155 The probability of working and full-time work is related to having additional children.198 Results from Frenette198 suggest that increases in the number of children results in a decline in the proportion of employed Canadian mothers. According to Scholder,23 _{the number of children} a woman has may influence her decision to participate in the workforce. The decision to work may be impacted by the perceived available time that is remaining after caring for children and completing associated household tasks.

The number of children in a household may impact the risk of childhood obesity. Several studies show that children without siblings are more likely to be obese compared to children with siblings.188,199–201 Hunsberger et al.200 found that singleton children were 1.52 times more likely (95% CI=1.34-1.72) to be overweight relative to children with siblings after

adjusting for known confounders. Formisano et al.199 found a negative dose-response relationship between the number of siblings and the risk overweight/obesity, where children with the greatest number of siblings (>2 siblings) had a significantly reduced risk of obesity relative to those who were an only child.

2.5.8 Household Income

Maternal employment is associated with household income in proportion to the magnitude of a woman’s contribution to family income. Mothers who are employed are able to increase the total income of their households with their earnings. In single-parent households, household income may consist entirely of a mother’s earnings, whereas in two-parent households, maternal work could be one of multiple sources of income.202

Household income and children’s overweight and obesity risk are associated.195,203,204 Compared to children living in low-income households, children living in high-income households have significantly lower odds of overweight and obesity.204 Living in medium income households and low income households is associated with a 1.8 and 2.8 times increased risk of obesity relative to children from high income households.205 Canadian data have shown that children from the highest income neighbourhoods are half as likely to be obese as children from the lowest income neighbourhoods.203 Similarly, annual household income is lower in families with obese children than in families with normal weight children.206

2.5.9 Maternal Immigrant Status

Participation in the labour force varies significantly between immigrants and Canadian-born counterparts. In Canada, 2009 data from the Labour Force Survey show that the rate of employment among Canadian-born individuals between 25 to 54 years was 82.9%, while among all immigrants, participation was lower, at 74.9%.207 An even larger gap exists between Canadian-born individuals and recent immigrants (≤ 5 years).207 In 2011, there was a 19.4 percentage point difference in the employment rate of Canadian-born individuals and recent immigrants.207 Gaps in wages between immigrants and Canadian-born individuals with equal education have also been demonstrated.207 In 2008, the weekly wages of recent

immigrants with a university degree were 70% of those earned by Canadian-born individuals with a university degree.207

A Canadian study shows that first-generation children experience greater weight gain relative to third-generation children.208 The higher rate of unemployment and lower wages observed among immigrants may contribute to the weight gain observed in first-generation children in Canada. Several aspects of maternal immigration may influence the risk of weight gain and overweight and obesity such as exclusive use of native language (which may limit access to healthy food and resources that promote healthy lifestyles),209 socioeconomic status, and country of origin.210 In contrast to evidence from Canada, results from studies conducted in the US and Italy show that while immigrant status and childhood overweight and obesity differ by ethnicity and generational status, immigrant status is associated with a lower risk of overweight and obesity compared to native-born individuals.211,212